A lens is a piece of transparent material shaped such that it bends parallel rays of light. The shape of a lens is a segment of a sphere, plane, or a cone (parabola, hyperbola, or ellipse). The bending of light causes the rays to cross, forming an image. This image can be larger or smaller than the actual object being viewed through the lens. The bending of light as it passes through a medium such as glass or plastic is called refraction. The early Greeks used lenses in magnifying glasses long before the ideas of refraction were understood. Today, lenses are used in many optical instruments, such as cameras, eyeglasses, and movie projectors.

Refraction is the bending of light that occurs when light travels through one medium into another. The amount a light ray is bent depends on the angle the ray makes with the interface between the...